The present invention relates to a method for doping a conjugated polymer. Polymers preparable according to the method of the present invention are provided.
Doping of conjugated polymers (polymers with pi-conjugated backbone structures and/or pi-conjugated pendant groups) with strong protonic acid (p-doping) or strong oxidising (p-doping) or reducing agents (n-doping) is well established in the literature. However, the doping proceeds readily to completion in the presence of a stoichiometric or excess amount of dopants. The chemical driving force for maximum doping is very high, so that it is difficult to arrest the doping level at an intermediate; is value. The system achieves the maximum doping with about 10-50% of the conjugated repeat units doped depending on the polymer system. For poly(p-phenylenevinylenes) and polyacetylenes, this is typically 10-20%; for polythiophenes, 20-30%; for polyanilines, 40-50%. This maximum level of doping imparts a high level of electrical conductivity of the order of 1-1000 S/cm to the polymers, depending on the nature and type of the polymers and dopants used, so that they become conducting polymers in the process. The bulk carrier concentration is then roughly of the order of 1020 /cm3 to 1021 /cm3.
However, this high level of doping is unnecessary or even undesirable for some applications. For example, for a 1-xcexcm thick film (which is typical of the vertical thickness of photonic structures) having a conductivity of 10xe2x88x926 S/cm, only a modest 1-V potential difference is required to drive a practical device current density of 10 mA/cm2 through the film thickness direction. Therefore, film conductivities of the order of 10-6 10xe2x88x922 S/cm (typical of the semiconducting range) are already sufficient for these films to be employed in semiconducting photonic structures such as distributed Bragg reflectors and waveguides.
Furthermore, when the films are doped to the maximum, such as achieved by straightforward exposure to strong acids or oxidants, their optical properties change in drastic ways owing to the formation of new sub-gap transitions-that change the refractive indices of the films and cause parasitic absorption of any emitted light. Both these factors are not desirable or acceptable for photonic applications. Therefore control of the bulk carrier concentration between 1017 /cm3 to 1020 /c3, at an intermediate doping-level at least about one order of magnitude less than the maximally-doped case, is crucial.
Applied Physics Letters, volume 73, Number 2, pages 253-255 (1998) reports a study of the Hall mobility and the carrier concentration of a conjugated polymer, namely polythiophene, as a function of the electrochemical doping level. The doping level of the polymer is changed by varying the oxidation potential i.e. by potentiometric control.
Synthetic Metals, 68, pages 65-70 (1994) is concerned with field-effect mobility and conductivity data obtained from two different amorphous organic semiconductors which can be doped to a range of different conductivities.
Synthetic Metals, 89, page 11-15 (1997) investigates the doping and temperature dependence of the conductivity of poly(p)-phenylene vinylene) (PPV).
Synthetic Metals, 55-57, page 3597-3602 (1993) investigates electrical conductivity of xcex1,xcex1-coupled dodecathiophene as a function of both dopant level and time.
Synthetic Metals, 30, page 123-131 (1989) discloses a relationship between acid strength and ionization potential of a conjugated polymer that will give a highly conductive doped complex.
Applied Physics Letters, volume 72, page 2147-2149 (1998) describes a doped hole transporting polymer. Differing levels of doping are realized by adjusting the co-evaporation rates of polymer and dopant material.
The methods used to achieve different levels of doping in the above systems are not satisfactory for controlling the doping level to such a degree so that a balance between optical and electrical property of the doped polymer can be struck.
In view of the above, there remains a need to develop a method for preparing polymers which are doped to a controlled, low or intermediate level which is both simple and cost effective It is envisaged that polymers doped to such a level will be particularly useful in devices such as those referred to,below in order to avoid the disadvantages associated with polymers that are doped to a high level. These disadvantages include intense sub-group absorptions, changes in the optical properties of the polymer and degradation of the photonic structure of the polymer. Using polymers that are doped to a controlled, low level or intermediate it will be possible to strike a balance between optical and electrical properties of an organic semiconductor when used in an optoelectronic device.
The present invention aims to provide a method for forming a conjugated polymer that is partially doped. The present invention further aims to provide a polymer preparable according to the method of the present invention and uses of such polymers.
Accordingly, the present invention provides a method for forming a conjugated polymer which is doped by a dopant comprising the steps of:
(a) adding a doping agent comprising a dopant moiety to a solution comprising the conjugated polymer or a precursor thereof and, optionally, a second polymer, the dopant moiety being capable of bonding to the conjugated polymer, precursor thereof or the second polymer;
(b) allowing the dopant moiety to bond to the conjugated polymer, precursor thereof or the second polymer to perform doping of the conjugated polymer, characterised in that the amount of doping agent added in step (a) is less than the amount required to form a fully doped conjugated polymer.
The present invention further provides a conjugated polymer that is doped to a controlled, low or intermediate level which is preparable according to the method of the present invention.
The present invention still further provides a photonic device including a polymer according to the present invention.
One embodiment of the present invention provides a method for forming a partially doped polymer material, comprising: adding a doping agent to the polymer or a precursor thereof, the doping agent being capable of bonding to the precursor or the polymer chain; and causing the doping agent to leave the precursor or the polymer chain to form a dopant capable of doping the polymer chain; wherein fewer moles of the doping agent are added than would be numerically sufficient to fully dope the polymer chain. Also, the present invention provides a partially doped polymer material formed by that method. Further, the present invention provides a device/structure (such as a photonic device) that includes such a material.
The conjugated polymer or its precursor is:
(i) derivatised with a controlled concentration (typically at the level less than 10-20% of the amount required for full doping) of a dopant moiety(ies) or its(their) precursor form(s); or
(ii) blended together with a polymer partner (the second polymer),, which may or may not be a conjugated polymer itself, which is derivatised with such moieties to give the equivalent dopant concentration.
Photonic structures are then fabricated from the partially doped polymer materials, including higher-order blends and composites, containing these modified conjugated polymers by film-forming techniques. A subsequent thermal, irradiation or chemical activation step may be required to generate the active dopant to dope the conjugated polymer.
In a first aspect of the invention, a method is provided to manipulate a precursor polyelectrolyte to give a controllable partially-doped conjugated polymer after elimination. The method involves replacement of a fraction of the counter-anions of the precursor polyelectrolyte by acid anions, such as sulfonates, phosphonates, phosphates, etc, of benzene, naphthalene and other organic derivatives while the precursor polyelectrolyte is in solution. These anions are converted during thermal elimination to the corresponding strong organic acids which are less volatile and more compatible with the conjugated polymer than the conventional anions, such as chloride, bromide and acetate. This leads to a higher retention of a strong acid that could favourably dope the polymer.
In a second aspect of the present invention, a method is provided for control over partial doping of a host conjugated polymer by blending with measured amounts of another substantially-miscible polymer (the xe2x80x9csecond polymerxe2x80x9d) which-is derivatised with a small fraction of dopant groups such as sulfonic acid, phosphonic acid or their precursors. The second polymer provides a means to distribute substantially homogeneously a controlled amount of dopant groups into the host conjugated polymer matrix. For this to occur, the second polymer must be co-soluble in the same solvent used to deposit the desired conjugated polymer film, and preferably not undergo phase segregation in the matrix. This can be achieved by derivatising to form a doped second polymer with a small fraction (usually less than 50 mol %) of the dopant groups. If the derivatisation reaction is carried too far, the material produced tends to be no longer soluble in the common hydrocarbon solvents used to solubilise the conjugated polymers because of strong interaction of the polar dopant groups.
In a third aspect of the present invention, a method is provided for control over partial doping of a host conjugated polymer in solution by derivatisation with measured amounts of the dopant moieties, such as sulfonic acid, phosphonic acid or their precursors thereby, in effect, creating a copolymer. The order of the reaction may be inverted. Either the polymer can be formed first and-then derivatised with a small mole fraction of the dopant; or the monomer could be derivatised first with the dopant group or its precursor and then incorporated at a small mole fraction into the primary conjugated polymer. The aim is to distribute substantially homogenously a controlled amount of dopant groups into the conjugated polymer matrix.